Well, this is a substitute for a person driven taxi. "Not being able to get a taxi" is a confusing reply, since this assumption is wrong. My question begore relates to what you think are the disadvantages, since you asked "Why is this better than a normal taxi?". I beliebe its the same, just no driver from the consumers point of view, is it not?
Less safe is up to the statistics. My guess is that its safe "enough" right now and only gets safer beyond humans. But the other point is more interesting: Flexibility. Since you don't need a driver, you can have a different kind of fleet management that - yes, I am an optimist - would be more available 24/7 than with human drivers.
See that's the thing, I don't see how it can ever be safer than all but the least competent human drivers.
I have yet to see an autonomous vehicle that could cope with anything other than a completely unobstructed road with everyone behaving themselves, and I can't see a way for autonomous vehicles to progress beyond that.
If you don't think taxis are available 24/7, you haven't met anyone that drives a taxi. Someone is always up and about.